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First planet found in star system next door

By Jacob Aron

(Image: ESO/L. Calçada)

Meeting the neighbours is normally easier than this. After years of searching, astronomers have finally spotted an Earth-mass planet in Alpha Centauri, the nearest star system to our own. Although the planet orbits too close to its parent star to host life, its discovery ups the chance of the system also hosting hospitable worlds.

Alpha Centauri looks like a single point of light from Earth, but it contains two bright stars that share a relatively close binary orbit, including one that looks a lot like our sun. This binary pair is in our cosmic backyard, about 4.3 light years away, spurring great interest in its ability to host planets.

“It is the most followed star in the search for planets,” says Xavier Dumusque of the Geneva Observatory in Switzerland. Finding such worlds, however, proved to be a challenge. So far, planet hunters have ruled out the presence of gas giants akin to Jupiter in Alpha Centauri. Finding smaller planets with the methods available takes patience.

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Using the La Silla Observatory in Chile, it still took Dumusque and colleagues four years to spot the planet around Alpha Centauri B, the smaller of the two stars.

The painstaking process included around 450 observations of the slight gravitational wobble the planet induces in Alpha Centauri B as it orbits the star. The task was made even more difficult by Alpha Centauri B reaching the peak of its solar cycle during the observations, increasing noise in the data.

Lava land

The team calculates that the new planet is 1.13 times the mass of Earth, which means it is likely to have a rocky composition. However, with a “year” of just over three Earth days, this rocky body is not our planet’s twin.

“The surface temperature must be hundreds – thousands – of degrees. There is perhaps lava floating on the planet,” says Dumusque. Still, planets tend not to be loners, so the Alpha Centauri system should have more. There’s a chance these undetected worlds are in the habitable zone, the region around a star most likely to support life as we know it.

“I’m really thrilled by the news, it’s just great,” says Javiera Guedes at the Swiss Federal Institute of Technology in Zurich, who in 2008 had predicted that it would be possible to spot a planet in Alpha Centauri, given the amount of data collected.

She says that if the finding holds up, further observations over a few more years could reveal a planet in the system’s habitable zone. “I just hope the result stands the test of time as more data is gathered,” Guedes says.

If you’re hoping to visit the neighbours, though, you may want to pack a few books for the trip. Even with the current record holders for the world’s fastest spacecraft, the Helios sun probes, the journey to Alpha Centauri would take 19,000 years – and that’s assuming you travel at top speed for the whole journey, which is unlikely.